Cosmarium cataractarum ( Raciborski 1889: 79 ) Eichler (1895: 60)

Coesel, Peter & Meesters, Koos, 2015, Taxonomic notes on Dutch desmids VII (new species, new names, new record), Phytotaxa 208 (1), pp. 55-62 : 58

Cosmarium cataractarum ( Raciborski 1889: 79) Eichler (1895: 60) (Figs. 4, 16–19)

This taxon was originally described by Raciborski (1889) as a variety of C. variolatum Lundell (1871: 41) . As main difference with respect to the nominate variety of C. variolatum Raciborski (l. c.) mentioned the occurrence, in var. cataractarum , of an ocellus-like cell wall pit in the centre of the semicell. However, when comparing the original descriptions of the two taxa, a number of additional differences may be noticed. Where C. variolatum in Lundell’s diagnosis is characterized by a semi-elliptic outline of the semicell, var. cataractarum is marked by a pyramidal shape resulting in a higher cell breath to length ratio (1.7 versus 1.2). In addition to that, although both taxa are marked by a scrobiculate cell wall, the cell wall pits depicted by Lundell (1871, pl. 2: 19) are distinctly coarser than those in Raciborski (1889, pl. 1: 3). Eichler (1895: 60) without any comment raised var. cataractarum to species level, but his point of view was hardly followed (e. g., Krieger & Gerloff 1965: 118). However, Šťastný & Kouwets (2012) on the basis of SEM photos convincingly demonstrated an essentially difference in the cell wall sculpture of the two taxa supporting the proposal of Eichler (1895) to consider C. cataractarum a separate species. Moreover, Šťastný & Kouwets (l. c.) ascertained a difference in ecology between the two taxa. Where C. variolatum is characteristic of acidic, meso-oligotrophic bog pools, C. cataractarum is particularly encountered in neutral-alkaline, meso-eutrophic water bodies.

Until rather recently, C. cataractarum was unknown to the Netherlands ( Coesel & Meesters 2007). Only in 2008 this taxon was detected for the first time, in a pasture pool near the village of Hilversum, in the province of Noord- Holland. Subsequently it was encountered on quite a number of other sites in the provinces of Overijssel, Noord- Brabant, Utrecht, Flevoland and Drenthe referring to habitats such as fen pools, reed beds and sand pits. All find sites can be characterized as neutral-alkaline, meso-eutrophic. Therefore, in the Netherlands C. cataractarum has turned out to be a widespread species, much more common than C. variolatum that is only known from a few acidic moorland pools ( Coesel & Meesters 2007).